Beyonce Knowles HBO documentary "Life Is But a Dream," is gearing up for its world premiere Saturday and the singer reportedly reveals that she wrote an emotional song for a baby that she miscarried before giving birth last year.

In the documentary, the 31-year-old singer revealed that the song for the baby that never made it out of her womb was the saddest one that she ever wrote.

"I flew back to New York to get my check up-- and no heartbeat. Literally the week before I went to the doctor, everything was fine, but there was no heartbeat," Knowles reveals in "Life Is But a Dream." "I went into the studio and wrote the saddest song I've ever written in my life. And it was actually the first song I wrote for my album. And it was the best form of therapy for me, because it was the saddest thing I've ever been through."

The entertainer's fansite Beyonce Lite took to Instagram to share the reported lyrics that Knowles wrote for her miscarried child.

"I guess love just wasn't enough for us to survive / I swear I swear I swear I tried / You took the life right out of me," Knowles reportedly wrote. "I'm so unlucky I can't breathe / You took the life right out of me / I'm longing for your heartbeat, heartbeat."

USA Today reported that the 31-year-old Grammy Award-winning singer also opened up about her thoughts concerning rumors that she allowed another woman to carry her baby.

"There's a stupid rumor. The most ridiculous rumor I think I've ever had about me. It's crazy. To think I would be that vain – I respect mothers and women so much and to be able to experience bringing a child into this world, if you're lucky and fortunate enough to experience that, I would never, ever take that for granted," Knowles said in "Life Is But a Dream." "It's the most powerful thing you can do. Especially after losing a child, the pain and trauma from that just makes it mean so much more to get an opportunity to bring life into the world. It seems like people should have boundaries."